1. Field of the Invention
The invention relates generally to a method of manufacturing semiconductor devices. More particularly, the invention relates to a method of forming a gate of a semiconductor device, wherein a loss of tunnel oxide film is minimized and damage to the semiconductor substrate is prevented.
2. Discussion of Related Art
A method of forming a gate using a high dielectric material as a dielectric layer has been used in a method of manufacturing a NAND flash memory device.
More particularly, a gate formation structure including a semiconductor substrate, a tunnel oxide film, a nitride film, an aluminum oxide (Al2O3) film, and polysilicon using aluminum oxide (Al2O3) is the high dielectric material is called a ″SANOS structure.
The gate formation process of the SANOS is briefly described below. A tunnel oxide film, a nitride film for a floating gate, an aluminum oxide (Al2O3) film, a polysilicon layer for a control gate, a tungsten silicide film, and a hard mask film are sequentially formed on a semiconductor substrate.
A photoresist film pattern is formed on the hard mask film. The tungsten silicide film, the polysilicon layer, the aluminum oxide film, and the nitride film are sequentially etched using the photoresist film pattern as a mask, forming a gate.
The gate etch process uses a dry etch process. After the nitride film is stripped, the etch process must be stopped at the tunnel oxide film.
In other words, a recipe in which the nitride film is easily etched and the tunnel oxide film is relatively less easily etched when etching the nitride film (e.g., the selectivity ratio of the nitride film to the tunnel oxide film is set to 10:1) must be used. It is, however, impossible to obtain such high selectivity ratio through prior dry etch techniques. Accordingly, a problem arises because the etch process is difficult to stop at the tunnel oxide film.
Therefore, the tunnel oxide film may be lost. Accordingly, there are problems in that the semiconductor substrate is also attacked and cell characteristics are degraded accordingly.